This invention relates to circuit breakers, and, more particularly, to a fault lockout protection incorporated in a circuit breaker.
In accordance with the terms used throughout the circuit protection industry, circuit breaker xe2x80x9cmaking capacityxe2x80x9d describes the ability to close a circuit breaker onto a high level short circuit current associated with a low power factor. The so-called xe2x80x9cmaking currentxe2x80x9d peak value, depending on switching transients and the point on the associated voltage wave closing angle for power factors between 0.15 and 0.20 is in the order of 2.309 to 2.183 times the rms current value. With a low power factor equal to approximately 0.04, the making current peak value can reach 2.663 times the rms current value. An rms fault current level of 100 KA, for example, would result in closing the circuit breaker onto a peak short circuit current of roughly 230 KA for 0.15 power factor. Most industrial-rated circuit breakers, however, are designed to interrupt, that is break, overload short circuit values that are significantly less than 150 KA. Accordingly, components within the circuit breaker operating mechanism are made of large mechanical structure to withstand the high mechanical and thermal stress associated with such short circuit currents. In addition, arcing must be more precisely controlled, the latching sequence must be more precise, and the degree of contact bounce that can be tolerated must be reduced to accommodate the short circuit currents.
Since the short circuit interruption-breaking current values occurring within an industrial electrical distribution system are much lower than peak closing short circuit current values, eliminating the need for a circuit breaker operating mechanism to close and latch onto a short circuit fault would subject the circuit breaker contacts and closing mechanism to considerably less mechanical and thermal stress. Therefore, eliminating the need for the circuit breaker to close and latch onto a short circuit fault would obviate the need for a stronger latching mechanism and would relax tolerances on the control of arcing, the latching sequence, and the degree of contact bounce.
In an exemplary embodiment of the invention, a circuit breaker with fault lockout protection includes a plurality of contacts forming part of an electrical distribution circuit. The contacts are separable to isolate a load side of the electrical distribution circuit from a line side of the electrical distribution circuit. The line side has a line voltage. A test current is induced on the load side of the electrical distribution circuit by a test voltage, which is less than the line voltage. A sensing device is arranged for sensing the test current in the load side of the electrical distribution circuit. The sensing device provides a sensed signal indicative of an electrical characteristic of the test current. A processor is arranged to detect a fault condition on the load side of the electrical distribution circuit in response to the sensed signal. The processor generates a fault lockout signal when the fault condition is detected. A fault lockout device receives the fault lockout signal and prevents closure of the plurality of contacts in response to the fault lockout signal.